1. Field of the Invention
The present invention relates generally to an HSDPA (High Speed Downlink Packet Access) communication system, and in particular, to an apparatus and method for transmitting/receiving control information on a shared control channel.
2. Description of the Related Art
In its earlier developmental stage, a mobile communication system focused on voice service only. Now, user demands and advanced mobile communication technology have developed a high-speed, high-quality wireless packet communication system to provide data service and multimedia service. Major efforts to deploy a 2Mbps or higher-speed, high-quality wireless packet service in a third-generation mobile communication system involve ongoing standardization of HSDPA and 1xEV-DV (Evolution-Data and Voice) in the 3GPP (3rd Generation Partnership Project) and 3GPP2 (3rd Generation Partnership Project 2). A fourth-generation mobile communication system is being developed to provide higher-speed, higher-quality multimedia service.
As its name implies, HSDPA provides high-speed packet data service to terminals via an HS-DSCH (High Speed-Downlink Shared Channel) and related control channels. To support HSDPA, AMC (Adaptive Modulation and Encoding) and HARQ (Hybrid Automatic Retransmission Request) have been proposed.
AMC is a technique for adapting a modulation and coding format based on the received signal quality of a UE (User Equipment) and the channel condition between a particular Node B and the UE to increase the use efficiency of the entire cell. Therefore, a plurality of modulation and coding schemes (MCSs) are defined for AMC. MCS levels are defined from level 1 to level n. In other words, the AMC is an adaptive selection of an MCS level according to the channel condition between the UE and the serving Node B.
In AMC, an MCS is changed according to a down-link channel condition, which is represented usually as an SNR (Signal-to-Noise Ratio) of a received signal in the UE. The UE feeds back the SNR to the Node B on an up-link. The Node B then estimates the down-link channel condition and selects an appropriate MCS based on the estimation. Modulation schemes under consideration are QPSK (Quadrature Phase Shift Keying), 8PSK, 16QAM (Quadrature Amplitude Modulation), and 64QAM, and coding rates under consideration are ¼, ½, and ¾. The Node B selects a high-order modulation scheme (e.g., 16QAM and 64QAM) and-a high coding rate (e.g., ¾) for a UE near to the center of the BS, that is, a UE in a good channel condition, and a low-order modulation scheme and a low coding rate (e.g., ½) for a remote UE, that is, a UE in a bad channel condition. As compared to a conventional MCS determining method relying on high-speed power control, interference is reduced and thus system performance is improved.
HARQ is a link control scheme for retransmission of an initial packet having errors in order to compensate for the errors. HARQ techniques include CC (Chase Combining), FIR (Full Incremental Redundancy), and PIR (Partial Incremental Redundancy).
In CC, the same packet as an initial transmission packet is retransmitted. A receiver combines the retransmission packet with the initial transmission packet stored in a reception buffer, thus increasing the reliability of coded bits input to a decoder and achieving an overall system performance gain. Since combining the same two packets is similar in effect to repetition coding, an average of an about 3-dB performance gain increase results.
In FIR, instead of retransmitting the same packet as an initial transmission packet, a data packet having only redundancy bits generated in a channel encoder is transmitted at a retransmission. Since the decoder decodes using new redundancy bits as well as the initial transmission packet, decoding performance is increased.
As described above, to support HSDPA, new techniques such as AMC and HARQ must be provided and new control information must be exchanged between a UE and a Node B. The new control information is delivered on an HS-SCCH (High Speed-Shared Control Channel), which will be described with reference to FIG. 1.
FIG. 1 illustrates a structure of the HS-SCCH in a typical HSDPA communication system. Referring to FIG. 1, the HS-SCCH includes TFRI (Transport Format and Resource Related Information), CRC (Cyclic Redundancy Check), and HARQ Information. The HS-SCCH has a period of 2 ms because a data unit delivered on the HS-SCCH is 3 slots (i.e., 2 ms). That is, the HS-SCCH has a TTI (Transmission Time Interval) of 2 ms.
The HS-SCCH delivers the following control information:
1) HS-DSCH (High Speed-Downlink Shared Channel) channelization code;
2) Modulation scheme (MS);
3) Transport block set size (TBSS);
4) Transport channel identity (TrCH ID);
5) UE-specific CRC;
6) HARQ Process ID;
7) New data indicator (NDI); and
8) Redundancy version (RV).
MS, TBSS, TrCH ID, and HS-DSCH channelization code are referred to as “TFRI information”. The TFRI information is delivered in the TFRI field. HARQ Process ID, RV, and NDI are referred to as “HARQ information” which is delivered in the HARQ field. The above control information will be described below in more detail.
(1) HS-DSCH Channelization Code
In the HSDPA communication system, down-link transmission resources are shared among a plurality of UEs. The down-link transmission resources include OVSF (Orthogonal Variable Spreading Factor) codes. It is under consideration to use 10, 12, or 15 OVSF codes when SF=16 and 20 OVSF codes when SF=32 in the HSDPA communication system. Assignment of OVSF codes in the HSDPA communication system will be described with reference to FIG. 2.
FIG. 2 illustrates an OVSF code tree with an SF of 16 in the typical HSDPA communication system. Referring to FIG. 2, each OVSF code is expressed as C(i, j) according to its position in the code tree. The variable i of C(i, j) represents the SF and the variable j represents the position of the OVSF code counted from the left, with the first position being numbered 0. For example, C(16, 0) indicates the first OVSF code from the left when SF=16. For an SF of 16, 10 OVSF codes C(16, 6) to C(16, 15) are assigned to the HSDPA communication system in FIG. 2. The 10 OVSF codes can be multiplexed for a plurality of UEs.
If there are HSDPA-supporting UEs A, B, and C, code multiplexing can be performed with 4 OVSF codes assigned to A, 5 OVSF codes to B, and the other one to C. Considering the amount of user data for each UE, a Node B determines the number of OVSF codes to be assigned to the UE and their positions in the OVSF code tree.
Use of 6 or 7 bits to represent information about channelization codes assigned to the HS-DSCH is under consideration in the present standardization work. For clarity of description, it is assumed that the HS-DSCH channelization code information is expressed in 7 bits.
(2) MS Information
As described before, a Node B selects an MCS adaptively according to a down-link channel condition between the Node B and a UE and tells the UE the MCS. Since the UE can determine the selected coding rate using TBSS, TrCH ID, HS-DSCH channelization ID, and MS, the Node B simply notifies the UE of the selected modulation scheme. In the following description, it is assumed that QPSK and 16QAM are available as modulation schemes and 1 bit is assigned to indicate the selected modulation scheme.
(3) TrCH ID
A transport channel is characterized by how information is transferred on a physical channel. In general, the transport channel is defined in terms of coding rate, channel encoding, transport block (TB) size, and the number of transmittable TBs during one TTI. If there are two different transport channels, it implies that they are different in terms of the above-described items. Because a plurality of transport channels can be time-division-multiplexed in an HS-PDSCH (High Speed-Physical Downlink Shared Channel), a UE must know which transport channel is active in the HS-PDSCH at a particular time. The transport channel is identified by its TrCH ID.
(4) TBSS
TBSS indicates the number of TBs transmitted during one TTI, so that a UE calculates the number of rate-matched bits in a physical layer. Rate matching refers to how repetition or puncturing is performed in the physical layer of a Node B. The rate matching and the TBSS are in such a relationship that the former is known from the latter. Therefore, a Node B does not transmit information about the rate matching to the UE. As described before, the TBSS is delivered in the TFRI field. Herein below, it is assumed that 6 bits are assigned to the TrCH ID and TBSS information.
(5) RV
If FIR is adopted as an HARQ technique, new redundancy bits are generated at a retransmission of an initial data packet. The Node B provides a redundancy bit combination indicator to the UE so that the UE can demodulate the data packet correctly. The redundancy bit combination indicator is an RV. It is assumed here that 4 puncturing patterns are available for redundancy bits and thus 2 bits are assigned to the RV.
(6) NDI and UE-Specific CRC
NDI indicates whether a data packet is initially transmitted or retransmitted. It is assumed that the NDI information is represented in one bit. UE-specific CRC makes a UE-specific ID more reliable. It is assumed that the UE-specific CRC is 12 or 16 bits. In the HS-SCCH slot format, the CRC field functions to check errors in the TFRI field, or in both the TFRI and HARQ Information fields.
(7) HARQ Process ID
Two techniques are used to increase HARQ efficiency. One is to exchange a retransmission request and a response for the retransmission request between the UE and the Node B, and the other is to temporarily store defective data and combine it with corresponding retransmitted data. In the HSDPA communication system, an n-channel SAW HARQ has been introduced to overcome the shortcomings of conventional SAW HARQ. In the conventional SAW HARQ, the next packet data is not transmitted until an ACK (Acknowledgement) signal is received for a current transmitted packet data. This implies that even though the next packet data can be transmitted, the ACK signal must be awaited. On the other hand, the n-channel SAW HARQ allows successive transmission of the next packet data without receiving an ACK signal for the current transmitted packet data, thereby increasing channel use efficiency. If n logical channels are established between a UE and a Node B and identified by specific time or their channel numbers, the UE can determine a channel on which a data packet has been transmitted at an arbitrary time point. The UE also can rearrange packet data in the right reception order or soft-combine corresponding packet data. A logical channel that delivers a particular packet is identified by an HARQ Process ID.
Table 1 below lists parameters delivered on the HS-SCCH and their sizes.
TABLE 1ParameterSize (bits)Channelization code set7MS1TrCH ID + TBSS6CRC16HARQ Process ID3NDI1RV2Total36
Now a description will be made of an HS-SCCH transmitter in the typical HSDPA communication system with reference to FIG. 3.
Referring to FIG. 3, before transmitting user data to a UE on an HS-DSCH, a Node B determines a channelization code 320 to be assigned to the user data through a code assigner 302, and an MS 318 and a coding rate through an MCS controller 304. Since the UE can determine the coding rate based on the MS 318, a TrCH ID & TBSS 310, and the channelization code 320, the Node B does not transmit information about the coding rate to the UE. An HARQ controller 306 determines an NDI 316, an HARQ Process ID 314, and an RV 312. A transport channel & block determiner 308 determines the TrCH ID & TBSS 310 for transmission of the user data.
A multiplexer (MUX) 322 multiplexes the channelization code 320, the MS 318, the NDI 316, the HARQ Process ID 314, the RV 312, and the TrCH ID & TBSS 310 to a bit stream in the HS-SCCH slot format. A CRC encoder 324 adds a CRC to the bit stream, and a serial-to-parallel converter (SPC) 326 converts the output of the CRC encoder 324 to an I bit stream and a Q bit stream.
Multipliers 328 and 329 multiply the I and Q bit streams by a predetermined spreading code COVSF, respectively. The multipliers 328 and 329 serve as spreaders. A multiplier 331 multiplies the output of the multiplier 329 by a signal component j. An adder 330 generates a complex signal by summing the outputs of the multipliers 328 and 331. A multiplier 332 multiplies the complex signal by a predetermined scrambling code CSCRAMBLE. Thus the multiplier 332 serves as a scrambler. A multiplier 334 multiplies the scrambled signal by a channel gain. A modulator 336 modulates the output of the multiplier 334 in the determined modulation scheme. An RF (Radio Frequency) processor 338 converts the modulated signal to an RF signal and transmits the RF signal in the air through an antenna 340.
FIG. 4 is a block diagram of an HS-SCCH receiver in the typical HSDPA communication system. Referring to FIG. 4, an RF processor 404 converts an RF signal received from the air through an antenna 402 to a baseband signal. A demodulator 406 demodulates the baseband signal in a demodulation method in correspondence with a modulation scheme used in the Node B. A multiplier 408 multiplies the demodulated signal by the same scrambling code CSCRAMBLE as used in the Node B. The multiplier 408 serves as a descrambler.
A complex to I & Q stream unit 410 separates the descrambled signal into an I bit stream and a Q bit stream. Multipliers 412 and 414 multiply the I and Q bit streams by the same spreading code COVSF as used in the Node B, respectively. The multipliers 412 and 414 serve as despreaders. A channel compensator 416 compensates for distortion possibly produced during signal transmission in the air.
A parallel-to-serial convert6er (PSC) 420 converts the channel-compensated signals to a serial signal. A CRC decoder 422 checks the CRC of the serial signal. If the signal is normal, the CRC decoder 422 feeds the signal to a demultiplexer (DEMUX) 424. The DEMUX 424 demultiplexes the CRC-checked signal into channelization code 426, MS 430, NDI 432, HARQ Process ID 434, RV 436, TrCH ID 438, and TBSS 440.
In the above-described HSDPA communication system, an initial transmission packet and a retransmission packet are transmitted with no distinction made between them. Control information about them is also transmitted in corresponding fields irrespective of initial transmission or retransmission, resulting in waste of radio resources. A puncturing pattern is preset for the initial transmission and thus there is no need for transmitting RV information to a UE at the initial transmission. The TrCH ID 438 and the TBSS 440 are not changed at the initial transmission and a retransmission. Therefore, it is unnecessary to transmit the TrCH ID and TBSS information at both the initial transmission and retransmission. It is because when an initial packet has errors, the packet is retransmitted on the same transport channel and the transport channel has the same TBSS. The indiscriminate data transmission wastes radio resources assigned to the control information. As a result, the overall system capacity is adversely affected. While the control information is delivered sequentially on the HS-SCCH at present, some control information may require processing with priority for demodulation of an HS-PDSCH signal related with the HS-SCCH signal. In this case, processing the HS-PDSCH signal might be delayed.